Bus hold is the ability of either an input pin or an I/O pin to retain the last valid logic state (voltage level) after the source driving it either enters the high impedance state, or is removed. There are four minimum current specifications that define Bus Hold. Two are sustaining currents that the node must source or sink without changing state, and two are overdrive currents that will cause the node to change state. IBHL is defined as “Bus Hold LOW sustaining current.” The bus hold circuit must sink at least this minimum LOW sustaining current when the input voltage is at the maximum level for a LOW logic state. IBHH is defined as “Bus Hold HIGH sustaining current.” The Bus Hold circuit must source at least this minimum HIGH sustaining current when the input voltage is at the minimum level for a HIGH logic state. IBHLO is defined as “Bus Hold LOW overdrive current.” An external driver must source at least IBHLO to switch the node from LOW to HIGH. IBHHO is defined as “Bus Hold HIGH overdrive current.” An external driver must sink at least IBHHO to switch the node from HIGH to LOW.
A prior art Bus-Hold circuit is shown in FIG. 1. The circuit of FIG. 1 includes transistors MP1, MN1, MPH1, MNH1, diode D1, resistor R3A, and input io. One of the problems with the prior art bus-hold solution in FIG. 1, is that it turns “on” and “off” the current source of the bus-hold circuit at the same time, causing switching current in the current source. It also does not have a current limiting resistor in the switching path of the current source. This causes a higher power dissipation capacitance while increasing the bus-hold current range.